The aliphatic esters and polyesters of substituted sterically hindered hydroxyhydrocinnamic acid are well-known as effective antioxidants for a wide variety of organic materials, protecting them from oxidative and thermal degradation. Many of these esters have gained wide spread commercial acceptance as phenolic antioxidants.
Batchwise transesterification methods for obtaining the instant ester compounds are known in the art. For example, methods of obtaining the octadecyl ester of dialkyl hydroxyphenylpropionic acid by ester exchange reaction of the methyl ester of dialkyl hydroxyphenylpropionic acid with octadecyl alcohol in the presence of an alkaline catalyst and the like, are known methods for obtaining higher alkyl esters of 3,5-dialkyl-4-hydroxyphenylpropionic acid by ester exchange reaction between alkyl esters of 3,5-dialkyl-4-hydroxyphenylpropionic acid and alkanols having higher alkyl groups. See, for example, U.S. Pat. Nos. 4,594,444 and 5,206,414. Japanese 57-136,548A teaches an analogous process, but utilizing aromatic sulfonic acids and cationic exchange resins as suitable transesterification catalysts. As noted hereinabove, these known methods involve batchwise-type chemistry, and are therefore readily distinguished from the instant continuous transesterification process for the production of substituted higher aliphatic esters of hydrohydroxycinnamic acids.
Japanese 02-180,851 and 02-180,852 teach a process for the production of B-(3,5-dialkyl-4-hydroxy-5-methylphenyl) propionate by reaction of 2,6-dialkyiphenol with an alkyl acrylate whereby the latter is continuously supplied in an amount of less then 1.0 mole per mole of 2,6-dialkylphenol in the presence of an alkali or alkaline earth 2,6-dialkylphenoxide as catalyst. No mention is made, however, for the continuous removal of the reaction products or of steady-state operation wherein reaction conditions are constant over time.
Reactive distillation methods are known in the art. For example, U.S. Pat. No. 5,536,856 relates specifically to the esterificafion of a carboxylic acid to form the carboxylic acid ester using a reactive column reactor having thereon an ion exchange resin containing sulfonic and/or carboxylic acid groups. U.S. Pat. No. 5,426,206 teaches the use of reactive distillation for the transesterification of a dialkyl carbonate with an aromatic hydroxy compound, such as phenol, in three successive reaction zones, each containing a catalyst such as a titanate ester, to form a diarylcarbonate. Also, the use of reactive distillation for hydrogenation of alkyl fatty acid esters to make the corresponding alkanol is known. However, there is no such method nor analogous method known by which to make the instant hindered phenolic ester compounds.
Indeed, one skilled in the art of antioxidants would not expect continuous transesterification by way of reactive distillation to lead successfully to the instant specialty chemical stabilizer compounds, where ultimate product stability is compromised if antioxidant quality is not high. "Batchwise" methods for preparing the instant compounds, which easily allow for stop-and-start sampling check points, have been historically preferred by industry. Batchwise operation conveniently provides for delays until analytical results become available, and thus allows for consistent and dependable product quality. In contrast, continuous reactive distillation is most often associated with commodity chemicals having lower product specification requirements. The most prevalent example is the manufacture of low molecular weight ethers, which are used as gasoline additives, where high product purity is not a stringent necessity.